Port vsock-rs to use rustix

[notgull]

vsock-rs currently uses a mix of nix and libc to handle
the underlying system calls. However, in addition to adding
a few extra dependencies, this means that there is a significant
amount of unneeded unsafe code in the project.

By porting to "rustix", we gain the following advantages:

• Most of the code in the project can be rewritten completely
  safely.
• We avoid going through libc. In other projects this has been
  benchmarked to lead to a 2.5% performance increase.
• On Linux, we reduce the number of dependencies the project
  pulls in. On other platforms, it stays the same.

This is a breaking change, since this crate exposes primitives from nix; specifically "VsockAddr". I've added a new "SocketAddr" type that exposes no types from any subcrates.

Blocked on bytecodealliance/rustix#1558

[qwandor]

Avoiding unsafe code is nice, but I don't think we should be avoiding going through libc. In particular on Android, where we use this crate in several parts of the platform, there are a number of places where libc has important platform logic (e.g. for tracking file descriptors) that shouldn't be skipped, and so we would like to avoid using rustix.

If there are some more safe wrappers in nix that we can use then I'm happy with moving to those rather than calling directly into libc.

[notgull]

there are a number of places where libc has important platform logic (e.g. for tracking file descriptors) that shouldn't be skipped

rustix has a feature named use-libc that makes it so it always uses libc, even on Linux. So there's still an option with rustix to use the libc backend. This just introduces the option to use raw Linux system calls.

If this still worries you, the libc backend is always used on Android for the reasons you bring up.

[qwandor]

Unfortunately as of bytecodealliance/rustix#1528 rustix uses linux-raw-sys on Android too, despite the discussion in bytecodealliance/rustix#1095. Practically, this means we can't use it in the Android platform without downstream patches, which will become more difficult to maintain if rustix continues to move in this direction. So my vote is against using rustix in vsock-rs.

[notgull]

If it comes to this, we can just enable libc in rustix for Android specifically.

[notgull]

Actually, my understanding of the above PR is that it isn't making Android use the raw system call backend. Even if use-libc is disabled, Android will still use the libc as the backend. This just brings in the linux-raw-sys crate for some constants.